The polymers are commonly in a form of pellets, granules and powder, herein referred shortly as pellets, when used for producing articles for a wide variety of end applications of polymers. Such pellets contain one or more polymer compositions and one or more further components, e.g. additives. Such further components may typically be incorporated i.a for protecting or for modifying the structure, property and/or function of 1) a pellet as such, which pellet can be an intermediate pellet product that is processed to further pellets or a final pellet for end use, or 2) a product produced using said pellets, which product can be an intermediate product which is further processed, e.g. crosslinked, to a final product or a final product, such as a cable layer or pipe.
Said further components can be incorporated into the pellet by adding component(s) to the polymer material before pelletising the composition. In some applications, see e.g. WO0038895 of Pirelli (=EP1150817) and EP1148518, one or more of said further components may be added after pellet formation by depositing the component(s) onto preformed pellet and allowing the component(s) to be absorbed inside the pellet matrix.
To obtain a desired effect of the added further component(s), it is important that said further component(s) is/are distributed as uniformly, i.e. homogeneously, as needed throughout a polymer pellet and also distributed uniformly between the pellets. As an example, e.g. isotropy and homogeneity are requirements in highly demanding field of electric cable manufacturing.
Thus, the method for incorporating and distributing said one or more further components within polymer pellets is very important for the performance of an article produced from said pellets.
It is thus a challenge to add, i.e. distribute, such one or more further components e.g. in a liquid medium onto the pellets. During said distribution step said further component(s) present in the liquid are typically allowed to impregnate into the pellets at a temperature below the melting point of the major polymer component of the polymer material of said pellet and then are subjected to a drying step including a further equalisation, i.e. homogenisation, step, if needed, for a predetermined time period in order to allow said further component(s) to diffuse into the pellets.
Mechanical mixing of the pellets e.g. during said distribution step can lead to the generation of polymer dust. Since the formed dust also absorbs said liquid, it may contribute to premature reactions and/or provide a non-homogeneous performance of the distributed further component(s). Thus, the dust must usually be removed in a subsequent step, for example by screening and elutriation. However, traces of dusty polymers often still remain as residues which affect the performance of the resulting article, such as an insulating layer.
As an example of said one or more further components that are incorporated to pellets by distributing the component(s) onto pellets, i.a. free radical generating agents can be mentioned. Free radical generating agents are conventionally used for modifying a polymer product via a radical reaction.
Free radical agents are used e.g. to initiate (a) a crosslinking reaction in a polymer, i.a. primarily a formation of interpolymer crosslinks (bridges) by radical reaction, (b) a grafting reaction in a polymer, i.e. introduction of compounds to a polymer chain (to backbone and/or side chains) by radical reaction, and (c) a visbreaking reaction in a polymer, i.e. modification of melt flow rate (MFR) of a polymer by radical reaction. These polymer modifications are well known in the art.
When added to a polymer composition, free radical generating agents act by generating radicals, typically by decomposing to radicals, under conditions which enable the radical formation.
Crosslinking of polymers, e.g. polyolefins, substantially contributes to an improved heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of a polymer. Crosslinked polymers, such as crosslinked ethylene homo- and/or copolymers, are commonly used as a layer material in wire and cable applications, such as insulating layer material, semi-conducting layer material and/or jacketing layer material. The crosslinked polymers are also widely used in other end applications of polymers such as in pipe applications.
In wire and cable applications a typical cable comprises at least one conductor surrounded by one or more layers of polymeric materials. In power cables including medium voltage (MV), high voltage (HV) and extra high voltage (EHV) said conductor is surrounded by several layers including an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. One or more of said layers are then typically crosslinked for the above reasons.
Peroxides are commonly used free radical generating agents used i.a. in the polymer industry for said polymer modifications.
It is also known to crosslink polyolefins, such as polyethylene, by introducing crosslinkable groups, such as hydrolysable silane groups, into the polymer by copolymerisation or grafting. The crosslinking of polymers with hydrolysable silane groups is carried out by so-called moisture crosslinking (water crosslinking) in the presence of e.g. a so-called silanol condensation catalyst, for instance carboxylates of metals, such as tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids.
Further components such as additives may naturally also be added onto pellets after pellet formation, such as antioxidants, light stabilizers, UV stabilizers, crosslinking boosters, scorch retardants, etc., as evident for a skilled person.
For example, it is important that a high voltage cable including crosslinked HV cable, comprises an isotropic and homogeneous insulating layer to obtain the high value of dielectric strength required for its operation.
The above mentioned WO 0038895 describes a method for introducing a liquid substance into polymer granules comprising spraying the substance onto continuously flowing granules in a static spraying chamber, passing the granules through a mixing chamber comprising static mixing means without moving mechanical elements to submit mixing thereof, and thereafter drying the granules in a drying chamber.
The granules flow in a substantially continuous manner by gravity through said spraying, mixing and drying chambers arranged in substantial vertical alignment. The spraying chamber may comprise a plurality of vertically arranged ducts. The static mixing chamber is designed to deviate the granules flowing in the central part of the mixing chamber towards the peripheral zone thereof, and the granules flowing in the peripheral zone towards the central zone thereof.
Known within the art is also the so-called continuous zig-zag blender, for instance supplied by Patterson Kelly, wherein a liquid substance is sprayed onto continuously flowing pellets. The zig-zag blender comprises an entrance part in the form of a drum section and thereafter a plurality of bends arranged in an essentially horizontal spiral manner. The whole equipment rotates 360° around its axis which results in movement of the pellets and thus mixing. The zig-zag blender is not self-cleaning. Moreover, the zig-zag blender comprises seals which are clear contamination sources since a seal in a rotating equipment is exposed to a continuous wear and therefore will be mechanically damaged with time and thus have limited use life.